Microelectronic integrated circuits based on patterned semiconductor materials are continuing to evolve towards devices with an extremely high density of circuit elements per unit volume. As the features of these devices are reduced to smaller sizes, the performance of the materials that constitute the device will critically determine their success. One specific area in need of advancement is the smaller conducting interconnects composed of materials with higher conductivity and greater mechanical integrity, which presently favors the use of copper (Cu), with twice the conductivity of aluminum and three times the conductivity of tungsten, as the material of choice. Another specific area in need of advancement is the electrical insulator used between the wires, metal lines, and other elements of the circuit. An insulating material that possesses a dielectric constant as low as possible, such as a dielectric constant (k) below the dielectric constant of silicon dioxide, about 3.9, has long been used in integrated circuits as the primary insulating material to avoid increased problems of capacitive coupling (cross-talk) and propagation delay.
Copper is much more susceptible to oxidation during processing. Copper also tends to diffuse into adjacent materials, including dielectrics. To use copper for interconnections, therefore, it is necessary to encapsulate the copper with barrier materials. It is common for a copper damascene process to use a barrier metal layer, such as titanium nitride, or tantalum nitride, deposited between a copper layer and an underlying material. After the copper layer is deposited, an encapsulation barrier layer, such as silicon nitride, tungsten, tungsten nitride, or titanium nitride, is deposited overlying the copper. U.S. Pat. No. 6,716,753, incorporated herein by reference, describes a method of nitridizing an upper surface of a copper layer to form a self-passivation layer comprised of titanium nitride. U.S. Pat. No. 6,130,157, incorporated herein by reference, describes a tungsten nitride encapsulation layer over copper interconnects. The problem with the copper encapsulation techniques used in the conventional arts is poor adhesion between the copper and the barrier material. Thus, an encapsulation layer serving as a barrier to diffusion of the copper layer into any overlying layers while improving adhesive characteristics between copper and any overlying layers is needed.
Physical vapor deposition (PVD) of the barrier metal layer has associated overhang, asymmetry, and sidewall coverage issues prior to copper electroplating, and it is becoming increasingly difficult to achieve continuous liner coverage on aggressive dual damascene structures. Liner coverage that is too thin or discontinuous on any surface will result in copper migration through such holes, causing reliability problems. Chemical vapor deposition (CVD) or atomic layer deposition (ALD) techniques that provide better step coverage of the barrier metal layer than PVD techniques, however, are generally more costly and encounter poor adhesion issues between copper and low-k dielectrics. There is therefore a need for a barrier layer that provides the desired adhesion between the low-k dielectric layer and the copper interconnects while still preventing oxidation and diffusion of the copper layer.
Wire bonding technologies for copper interconnects often utilize an aluminum layer to cap the exposed copper wire bond pad. This aluminum cap is added to allow use of the same wire bonding tools and processes used in aluminum interconnect technologies. When the aluminum metal is deposited on copper, however, most of the aluminum is consumed when reacting with the underlying copper, forming CuAl2. A barrier layer of tantalum nitride inserted between the underlying copper pad and the top aluminum pad is necessary to prevent the reaction between these metals, but the accompanying problem of poor adhesion that exists between tantalum nitride and copper must be overcome. U.S. Pat. No. 6,350,667, incorporated herein by reference, describes an adhesion aluminum layer inserted between tantalum nitride and copper for improving adhesion in the pad metal stack structure. The present invention is also directed to a copper interconnection that is protected from copper oxidation, and provides improved surface adherence qualities for the top-level copper layer and the wire-bonding aluminum pad.